ARROW-4459: [Testing] Add arrow-testing repo as submodule

This PR adds the arrow-testing repo as a submodule in the directory `testing`.

Author: Andy Grove <andygrove73@gmail.com>

Closes apache#3547 from andygrove/ARROW-4459 and squashes the following commits:

522ac26 <Andy Grove> Add arrow-testing submodule

Author: andygrove

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "cpp/submodules/parquet-testing"]
 	path = cpp/submodules/parquet-testing
 	url = https://github.com/apache/parquet-testing.git
+[submodule "testing"]
+	path = testing
+	url = https://github.com/apache/arrow-testing